Heretofore, almost all laminated structures have been manufactured using thermoset resin solution impregnation techniques as taught, for example, by McCaskey et al., in U.S. Pat. No. 4,061,823. In the usual procedure, thermoset resins in the B-stage, i.e., still fusible and soluble, are dissolved in solvent, and the resulting solution used to completely impregnate fibrous woven substrate material. Then, the solvent is driven off in a long, specially designed oven. After leaving the oven, the B-staged resin is distributed throughout the interior of the fibrous material, to provide a prepeg substrate.
The next step in laminate manufacture is to cut the resin impregnated prepreg to desired sheet dimensions, and subject a superimposed plurality of said prepreg sheets to heat and pressure, usually in a flat bed press, at about 150.degree. C. and 1,500 psi. This consolidates the sheets into a bonded, completely cured, unitary structure. The purpose of the above procedure is to bring an even distribution of thermoset resin throughout the interior of the sheets of fibrous substrate material, and intimate contact, or bond, between each fiber filament in such substrate material within the matrix resin.
The usual method described above has certain disadvantages, however. One disadvantage is the expense of operating the special drying ovens and the disposal of solvent vapors generated in the ovens. Yet other disadvantages are the required use of strong, and expensive, woven fabric substrate material, which is able to resist the tension of pulling through the oven, and problems associated with storage of thermost resin impregnating solutions, for a commercially acceptable time period, without their starting to gel.
Renfrew et al., in U.S. Pat. No. 2,199,597, tried to solve some of these laminate manufacturing problems, by first impregnating or coating a fibrous substrate, such as cotton fabric or absorbent paper, with an adhesive, and then applying a thermoset phenolic resin powder, which adheres to the adhesive at the top surface of the substrate. A plurality of such powder-coated sheets are then assembled and heat and pressure consolidated in a flat bed press, at about 140.degree. C. and 1,500 psi. The problem with this process, however, is that the thermoset resin would fail to completely impregnate the fibrous substate due to the blocking effect of the adhesive.
Guettinger et al., 7th Annual Conference Reinforced Plastics Composite Insitute, Society of the Plastics Industry, Inc., January 1982, Session 10-D, pp. 1-5, while analyzing the economies of continuous lamination, described continuous double belt press blank formation, where glass fibers, carbon fibers, woven mats, aramid fibers, wood flour, and glass spheres are introduced into polypropylene, polyester or polyamide matrices. The blanks are then slit or trimmed in the product flow direction, for use in subsequent processes, such as molding to form automotive components. In one process, the resin, in powder form, is first extruded through a slotted die to provide a sheet of plastic material which is passed through the press between glass fiber mats. The resin sheet material can also be fed from a roll as a continuous room temperature sheet. This process, however, has the disadvantage of requiring expensive, initial melt processing of the resin powder to continuous sheet form before feeding into the continuous laminating process. Also, the use of a continuous sheet of resin will not allow complete resin impregnation into the fibrous mats.
What has been long needed is an efficient process for laminating either a nonwoven, loosely felted substrate, or a fibrous woven substrate; which allows good resin impregnation of the substrate, yet eliminates heat energy requirements, solvent pollution, and impregnating resin pot life problems.